Tweaks

Author: henricasanova

topic_basics_of_distributed_memory_programming/sequential_step1.html

@@ -5,32 +5,72 @@
   <div class="item">Allocates an 1-D <code>char</code> array of (n*840)*(2*n*840)*3 elements. This array is used to
     store an
     image with height (in pixels) of n*840, width (in pixels) of 2*n*840, where each pixel is encoded using 3 bytes (RBG
-    values).
+    values). By passing different values of n we can scale the resolution of the image, with n=1 being the
+    smallest resolution (840 x 1680).
   </div>
   <div class="item">Fills this array with pixel values corresponding to the Julia set.</div>
 </div>
 
 <p class="ui">
   To make the above easier you are provided with one helper C function, <code>compute_julia_pixel()</code>, to which
   you pass the (x,y) coordinates of a pixed, the (width, height) dimensions of the image, a "tint" float (pass 1.0
-  for now), and a pointer to 3 contiguous bytes. These bytes are set to appropriate RGB values (Red Green Blue) for
-  displaying the Julia set.
+  for now), and a pointer to 3 contiguous bytes (<code>unsigned char</code>). These bytes are set to appropriate
+  RGB values (Red Green Blue) for displaying the Julia set.
   <a target="_blank" href="{{site.baseurl}}topic_basics_of_distributed_memory_programming/compute_julia_pixel.c">[Download
-  source of compute_julia_pixel()]</a>
+    source of compute_julia_pixel()]</a>
 </p>
 
-<div class="ui accordion styled fluid">
+<div class="ui accordion fluid">
   <div class=" title">
     <i class="dropdown icon"></i>
-    See the source of compute_julia_pixel()
+    See the source of compute_julia_pixel()...
   </div>
   <div class=" content">
-    <div class="ui container ">
+    <div class="ui raised container segment ">
 
-        {% highlight Java %}
-        {% include_relative topic_basics_of_distributed_memory_programming/compute_julia_pixel.c %}
-        {% endhighlight %}
+      {% highlight Java %}
+      {% include_relative topic_basics_of_distributed_memory_programming/compute_julia_pixel.c %}
+      {% endhighlight %}
     </div>
   </div>
 </div>
 
+<br>
+
+<p class="ui">
+  The program must store the pixels of the 2-D image into a <b>1-D array using a <i>row-major scheme</i></b>.
+</p>
+
+
+<div class="ui accordion fluid">
+  <div class=" title">
+    <i class="dropdown icon"></i>
+    See more details about this "row-major" business...
+  </div>
+
+  <div class="content">
+    <div class="ui container raised segment">
+      <h4 class="ui header">Storing 2-D data is a row-major 1-D array</h4>
+      <p class="ui">
+        You may find the use of a 1-D array to store the pixels surprising, since after all the image is 2-D. However, it is
+        common to use 1-D arrays, especially when they are dynamically allocated. One option would be to allocate an array
+        of pointers to
+        1-D arrays, making it possible to access elements with the convenient syntax <code>Array[i][j]</code>.
+        But this is typically a bad idea as it reduces locality (i.e., the efficient use of the cache), which in turns harms
+        performance.
+      </p>
+      <p>Therefore, we instead store 2-D data into a 1-D array of contiguous elements. The "row-major" scheme consists in storing rows in sequence. In other
+        terms, if the width of the image is <code>N</code>, then pixel (i,j) is stored as <code>Array[i * N + j]</code>. This is actually
+        the scheme used by the C language to store 2-D arrays.
+      </p>
+    </div>
+  </div>
+</div>
+<br>
+
+
+Now that the program computes the Julia set pixels as a 1-D array of 3-byte elements, it would be nice to see them (if only to check
+that they're set correctly). So move on to Step #2 (next tab)...
+
+
+